First thing: Russia is massive, both in a sense of population and a sense of land. A lot of jokes about Russia sending swarms of men to battle have been made, but they did have quite a large population of people who could fight. Military casualties for the USSR in World War 2 ended up at 8 million. 8 million soldiers, not total, but killed/taken captive. The Soviet Union and Russia in general had a great potential to trade casualties and turn wars into attrition filled slogs, and the land thing is similar. Czar Alexander, who faced off against Napoleon, just kept on retreating with a scorched earth policy, which devastated Napoleon's forces, which were accustomed to surviving off of forage (living off the land). Speaking of forage, another issue which might halt the ambitious conqueror is supply lines. Large land mass means that logistics will be stretched. Supply lines need to established, and since these lines are manned by human beings, the same people moving supplies need supplies, which means that it can get difficult to deliver supplies far afield. Add to this is the phenomena known as rasputitsa, where unpaved roads turn to a muddy slog in autumn and spring. This makes maintaining logistics even harder. In company of General Mud is the famed General Winter. That is not to say that every invasion of Russia failed due to winter. Napoleon's forces were already suffering from lack of supply when winter came in, and the Soviets were fiercely resisting Germany when winter came in and made problems of low supply worse. You need specialized supplies (like heavy coats and winter gear) to survive in winter conditions, and winter makes prosecuting an offensive even harder. Now you're charging some defensive position while cold and tired, and the defensive position might have some form of shelter where the enemy can stay warm (or warmer), the wounded need to be evacuated faster, and infantry movement is made much less efficient over snow as compared to flat ground (that's why we have skis and snowshoes, kids). For an example of Russia ruining campaigns other than the Nazis or Napoleon, look at the boy king Charles XII. He was ganged up on by an alliance of Denmark-Norway, Poland, and Russia, and managed to handle the former two fairly well (the puppet king he installed in Poland would eventually lose control, but stories for another time), before turning to Russia. For a while he did extremely well, routing forces several times his size at battles like Narva (~12,000 Swedes v 37,000 Russians, decisive Swedish victory), Neva (2,000 Swedes v 8,000 Russians, Swedish victory), and Lesnaya (11,500 Swedes v ~23,000 Russians, Swedish victory) to name a few. This makes a very important point: Russia is not invincible. It has advantages, but it can lose battles just like anyone else, and while it has many strategic advantages, Russians have lost wars. They fell to the Mongols in 1240, Moscow fell to the Golden Horde in 1382, the Ingrian war in which the Swedes made great territorial gains in 1617 during the Times of Troubles, and the Crimean war in 1856. Of course, not all of these wars resulted in the annexation of large swaths of Russian territory or anything, but they can be invaded as well as anyone else. Monstrous tangent aside, I was talking about Charles XII, and how he was doing well against Russia at first. Scorched earth tactics made the Swedish advance harder, and expected reinforcements did not arrive (not due to bad logistics in this case, the reinforcing army was ambushed and expected help from Ukrainian cossacks never arrived). This in company with a particularly cold winter in 1709 shrunk Swedish forces even more, this would be followed by the disastrous battle of Poltava, where the Swedish force was hammered, losing several thousand men, and in company with a massive Swedish surrender at Perevolochna marked the end of the Swedish invasion, and in a greater sense, the Swedish empire. You could make a point about how the well drilled Caroleans of the Swedish Army could fight off larger numbers of Russians, but were much harder to replace, and I think that is important to mention as well. A similar trend showed up in WW2. The Soviets could produce T-34s at staggering rates, in part by researching how long they lasted in battle, and cutting down on costs by not designing the tanks to last any longer than that. T-34s were pretty good tanks, and even if you argued that Germans had perfect Wunderwaffe tanks (which they did not, they were prone to frequent breakdowns) they couldn't be produced in numbers to match Russia's. It became a war of attrition which the Germans just could not handle. Not enough men, not enough fuel, not enough tanks.